Acute Kidney Injury (AKI) Biomarker

REVIEW ARTICLE
Acute Kidney Injury (AKI) Biomarker
Sri S. Adiyanti, Tonny Loho
Department of Clinical Pathology, Faculty of Medicine, University of Indonesia – Cipto Mangunkusumo Hospital.
Jl. Diponegoro no. 71, Jakarta 10430, Indonesia. Correspondence mail: theayukari@yahoo.com
ABSTRAK
Ginjal mempunyai kapasitas yang mengagumkan terhadap gangguan fungsi untuk waktu yang lama.
Sensitivitas sel ginjal secara individual terhadap trauma sangat tergantung pada jenisnya, posisinya dalam
nefron, vaskularisasi lokal, dan asal trauma/gangguan. Kerusakan ginjal yang terjadi merupakan hasil dari
disfungsi sel, kematian sel, proliferasi, inflamasi, dan pemulihan.
The Acute Kidney Injury Network (AKIN) mendefinisikan AKI sebagai “kelainan fungsional dan struktural
atau tanda terjadinya kerusakan ginjal termasuk kelainan pada darah, urin, atau jaringan dan pencitraan yang
telah ada selama kurang dari 3 bulan”. Sistem rujukan yang paling banyak digunakan adalah kriteria RIFLE
(Risk, Injury, Failure, Loss, End-stage Kidney Disease). Biomarker ideal untuk AKI haruslah murah, cepat dan
mudah diukur, teliti dan akurat, mampu menentukan beratnya disfungsi, spesifik untuk ginjal, meningkat di awal
terjadinya disfungsi, dan mempunyai sensitivitas dan spesifisitas yang tinggi. Usaha untuk mendeteksi AKI pada
fase awal telah menghasilkan beberapa biomarker yang menjanjikan seperti KIM-1, NGAL, IL-18, Clusterin,
dan sebagainya. Cystatin C merupakan biomarker yang bekerja pada fungsi filtrasi glomerulusnya, sedangkan
β2-microglobulin, α1-microglobulin, NAG, RBP, IL-18, NGAL, Netrin-1, KIM-1, Clusterin, Sodium Hydrogen
Exchanger Isoform dan Fetuin A merupakan biomarker yang bekerja pada fungsi reabsorbsi tubulusnya.
Kata kunci: AKI, biomarker, RIFLE.
ABSTRACT
The kidney has a remarkable capacity to withstand insults for an extended period of time. The sensitivities
of individual renal cells to injury vary depending on their type, position in the nephron, local vascularization,
and the nature of injury. The resulting kidney injury is a product of the interplay between cell dysfunction, cell
death, proliferation, inflammation, and recovery.
The Acute Kidney Injury Network (AKIN) defined Acute Kidney Injury (AKI) as “functional and structural
disorder or signs of renal damage including any defect from blood and urine test, or tissue imaging that is less
than 3 months.” RIFLE (Risk, Injury, Failure, Loss, End-Stage Kidney Disease) criteria is the most frequently
used system. Ideal biomarker for AKI should be affordable, quick and measurable, precise and accurate, with
prognostic ability to define severity of renal dysfunction, specific for renal, increase in the early stage dysfunction,
with high sensitivity and specificity. Efforts to detect AKI in the earlier stage has resulted in some promising
biomarkers such as KIM-1, NGAL, IL-18, Clusterin, etc. Cystatin C is a biomarker for glomerular filtration
function, while β2-microglobulin, α1-microglobulin, NAG, RBP, IL-18, NGAL, Netrin-1, KIM-1, Clusterin,
Sodium Hydrogen Exchanger Isoform and Fetuin A are biomarkers for tubular reabsorption function.
Key words: AKI, biomarker, RIFLE.
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Vol 44 • Number 3 • July 2012
INTRODUCTION
Conceptually, acute renal failure (ARF) is
defined as a rapid decrease of glomerular filtration
rate (GFR), which can go over hours to weeks
and it is usually reversible. The traditionally used
term ARF often is used in reference to the subset
of patients admitted in the intensive care unit
that need immediate dialysis support. Increases
in serum creatinine are associated with risk for
mortality; therefore, kidney dysfunction should
be captured for early detection and intervention.
The clinical spectrum of decline in GFR is broad
and any minor deterioration in GFR and kidney
dysfunction should be diagnosed as the presence
of kidney damage. For that reason, the term
ARF is replaced by that of acute kidney injury
(AKI) and the term ARFpreferably should be
restricted to patients who have AKI and need
renal replacement therapy (RRT).1
Acute Kidney Injury (AKI) is diagnosed with
progressive rise in serum creatinine over several
days, which may or may not be accompanied
with oliguria. Serum creatinine changes can differ
from actual GFR changes. Clinical diagnosis
is often delayed due to lacking signs of renal
function deteriorations. Delayed diagnosis is a
problem because lacking signs of renal function
deteriorations. The detection of a reliable biomarker
for early diagnosis of AKI would be very helpful
in facilitating early intervention, evaluating the
effectiveness of the therapeutic intervention, and
guiding pharmaceutical development.1,2
Recent studies reported that routine renal
function test based on serum creatinine, blood
urea nitrogen and urine production were outdated
because they failed to identify early stages of
renal dysfunction and structural injury.3 Unlike
serum troponin in myocardial infarction, increase
of serum creatinine is not directly correlated with
tubular disturbances in AKI but it is correlated
with filtration function. Creatinine changes are
not specific because it can also occur as a result
to non-renal etiologies, such as muscle mass and
nutrition intake.4
When a patient has angina pectoris, measuring
biomarker such as troponin which is released by
damaged myosin will be identified directly as
acute myocardial injury; therefore, the same
principle should be used in defining biomarker
in AKI or we can say angina renalis. Most AKI
were asymptomatic.4
Biomarker Acute Kidney Injury
Because of the vital importance of earlier
targeting of therapies, many markers have been
explored for early diagnosis of AKI. Although the
initial studies on some molecules such as tubular
enzymes, growth factors, adhesion molecules,
and some cytokines were promising, however,
there are inadequate sensitivity and specificity
to advocate clinical use.2
The objective of our manuscript is to discuss
further about AKI and its pathophysiology and
some previous biomarkers that have been used
and recent biomarker which still needs further
studies, which expectedly can be used to detect
early stage AKI.
ACUTE KIDNEY INJURY (AKI)
The kidney has a remarkable capacity to
withstand insults for an extended period of time.
The sensitivities of individual renal cells to injury
vary depending on their type, position in the
nephron, local vascularization, and the nature of
injury. The resulting kidney injury is a product
of the interplay between cell dysfunction, cell
death, proliferation, inflammation, and recovery.
This sequence of events is determined by time to
diagnose and affect the cause of kidney failure in
the event; therefore, sensitive and specific tests
for early diagnosis of kidney injury are extremely
needed.3
The Acute Kidney Injury Network (AKIN)
defined AKI as “functional and structural disorders
or signs of renal damage, including defect in
blood and urine test or tissue imaging, which
exist less than 3 months”. AKI could be caused
by decrease of renal or intra-renal perfusion,
obstructive or toxic renal tubular disorders,
tubular-interstitial inflammation and edema, or
decrease of glomerular filtration capacity.
a. Pathophysiology of AKI
-- Vasoconstriction
-- Tubular cell desquamation
-- Intraluminal tubular
obstruction resulting in
‘tubular backleak’
-- Production of local
inflammatory mediators,
resulting in interstitial
inflammation, small vessels
obstruction, and local
ischemia.
Figure 1. The pathophysiology of AKI
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b. Cellular level
Figure 2. The cellular mechanism of AKI
The mechanism of pathophysiology may
contribute to the development of AKI, which
causes ischemia or toxic damage, i.e. (a) disruption
to renal perfusion which compromise renal autoregulation and caused renal vasoconstriction,
(b) tubular dysfunction and cell death due to
apoptosis and necrosis (c) cell desquamation
which contributes to intra-tubular obstruction
(d) metabolic disturbances which cause transport
disorders, hence disrupting tubular-glomerular
balance, and (e) production of local inflammatory
mediator causing interstitial inflammation and
vascular congestion. In cellular level, there are
loss of cytoskeletal integrity and cell polarity,
with displacement of adhesion molecule and other
membrane protein such as Na+K+ATPase and beta
integrin, loss of proximal tubular brush border, like
apoptosis and necrosis (Figure 1 and 2).
With continuing damage, both viable and
non-viable cell were desquamated leaving
space of basal membrane as the only barrier
between filtrate and peritubular interstitium.
Such condition may result in a backleak or
leaking of the filtrate back, especially if there
is an increase of intratubular pressure, which
causes intratubular obstruction and causing
interaction of cellular debris with protein such
as fibronectin in the lumen. Epithelial damage
may induce secretion of inflammatory and
vasoactive mediators, which may worsen the
vasoconstriction and inflammation.5
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RIFLE CRITERIA
The need to describe AKI precisely and
sensitively has caused development of a
multidimensional AKI classification system,
which describes the severity of AKI. The most
frequently used reference system is the RIFLE
criteria (Risk, Injury, Failure, Loss, End-stage
Kidney Disease). RIFLE acronym describes 3
severity stages of acute renal failure (Risk, Injury
and Failure) and 2 output variables (Loss and End
Stage Kidney Disease). Unique characteristics
of the RIFLE classification include the presence
of 3 variables to describe the severity of renal
dysfunction based on serum creatinine level,
GFR changes or its duration, and the severity
of reduced urine production. The advantages of
using RIFLE criteria is that we could establish
diagnosis at the stage of preventable renal
dysfunction (risk stratification), or when the renal
injury has occurred, and the confirmed kidney
failure.1,6
The first stage of RIFLE criteria (Risk) might
be the most important stage, as in this stage, the
positive test results should raise the physician’s
awareness of the risk of renal damage, at the
moment when it is still reversible with prevention
or therapeutic intervention. Parameters used for
screening should have high sensitivity, affordable
and easily accessed. The risk category is defined
when the serum creatinine level increases two
fold or when there is a decrease of urine output
Vol 44 • Number 3 • July 2012
Biomarker Acute Kidney Injury
Table 1. The Classification of AKI5
Stage/
Category
Serum Creatinine
Criteria
Urine Output
Criteria
1 (Risk)
Increased serum
creatinine level ≥0.3
mg/dl or 150% - 200%
increase from baseline
Urine production
<0.5 ml/kg/hour
for >6 hours
2 (Injury)
Increased serum
creatinine level
>200%-300%
Urine production
<0.5 ml/kg/hour
for > 12 hours
3 (Failure)
Increased serum
creatinine level from
baseline (or ≥4 mg/
dl) (acute increase of
≥0.5 mg/dl)
Urine production
<0.3 ml/kg/hour
x 24 hours or
anuria in 12
hours
Loss
Persistent ARF =
complete loss of renal
function >4 weeks
ESKD
End Stage Kidney
Disease >3 months
<0.5 ml/kg/ hour for at least 12 hours. Now this
risk definition has been broaded including the
increase of absolute serum creatinine level, which
is 0.3 mg/dl (26.5 µmol/L) or more. An analysis of
RIFLE criteria in 5383 critical patients has shown
that 1510 (28%) patients were in risk stage, 840
(56%) patients showed further progressivity
towards more severe RIFLE stage, which means
that the criteria has a specific reason to detect
the differences between functional disorder
(vasoconstriction due to renal hypoperfusion) and
structural disorder (acute tubular necrosis). For
the risk category, the possibility of AKI should
be observed in post-surgery patients, and patients
with nephropathy due to toxic substances such as
contrast media. For pre-renal etiology/ or injury
category, tubular function is still intact and the
decrease of filtration is correlated to sodium
reabsorption in the tubulus, which could be
observed in earlier stage of obstruction, such as in
acute glomerulonephritis, pigment nephropathy,
and ARF due to contrast media. At the failure
stage, such category is defined as conditions
indicated for renal replacement therapy including
volume overload, hyperkalemia, metabolic
acidosis, and the presence of excessive uremia.
At the stage of loss of renal function, it is
characterized by survival of ARF patients with
improving kidney function; therefore, the renal
replacement treatment is no more necessary
or they have longstanding needs for renal
replacement therapy (more than 4 weeks), while
ESRD stage is characterized by irreversible renal
function, especially in patients who had previous
history of chronic kidney disease.1
AKI BIOMARKERS
AKI biomarkers can be components of serum
or urine. Urine biomarkers are quite promising
to detect early AKI, hence it can be anticipated
earlier; therefore, it could be useful for early
diagnosis, identification of mechanism disorders,
and determination of location and severity of
dysfunction.5
The term biomarker (acronym for biological
marker) was first described in 1989, which means
measurable indicator for a specific biologic
condition and for specific disease process. In
2001, biomarker definition were standardized
to be a characteristic that can be measured
and evaluated as normal biological process,
pathological process or pharmacological response
to therapeutic intervention. Moreover, the Food
Drug and Administration (FDA) use biomarker
term to describe any diagnostic indicator that
can be measured and used to assess any risk or
disease. The ideal biomarker for AKI should be
affordable, fast and easy to measure, precise and
accurate, and able to determine the severity of
dysfunction, specific for kidney, increase in the
early stage of dysfunction, with high sensitivity
and specificity.7
There are still a lot of opportunities to develop
a biomarker that can actually detect earlier stage
of tubular cell disturbances before it disrupts
renal filtration capacity. Several biomarkers have
been found, some seem to be promising, such as
kidney injury molecule-1, netrophil gelatinaseassociated lipocalin, IL-18, sodium/hydrogen
exchange form 3, N-acetyl-β-D-glucosaminidase,
and Netrophil gelatinase-associated lipocalin and
kidney injury molecule, these biomarkers have
increased level in urine at a very early stage
(in 2 hours) after dysfunction occurs, which is
followed by IL-18 in 12 hours.1
b2-MICROGLOBULIN
β2-microglobulin (β2-M) is a 11.8 kDa protein,
which is a light chain of major histocompatibility
class (MHC) I expressed on the cell surface of
every nucleated cell. β2-M dissociates from
the heavy chain on cellular arrangements and
entering circulation as monomer. β2-M is
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Figure 2. Kidney injury continuum and biomarker roles5
filtrated by glomerulus and almost all of them
have undergone reabsorption and catabolism by
proximal tubular cell, the process which might
be disrupted in AKI.
Increase β2-M excretion in urine has been
reported as early signs of tubular dysfunction
due to many causes, including nephrotoxic
substances exposures, cardiac surgery, and
kidney transplantation, which precedes 4-5
days before the serum creatinine level increases.
However, disadvantages of β2-M as a biomarker
is its instability in the urine and fast degradation
in room temperature and urine with pH <6.0.5
α-1 MICROGLOBULIN
α-1 microglobulin (α1M) is a protein
synthesized in the liver, that is half of protein
circulated bound to IgA complex. The free
forms are filtrated by glomerulus and undergo
reabsorption by proximal tubular cell. Unlike
β2-M, α1M form is stable in urine with vast range
250
of pH as found in routine clinical examination,
therefore, it is a more preferred biomarker. α1M
is also a sensitive biomarker for proximal tubular
dysfunction, even for early stage of dysfunction,
which no histological damage could be seen.5
CYSTATIN C
One of alternative methods to detect early
GFR decrease is by monitoring serum cystatin
C, level, which is produced constantly by all
nucleated cells and are filtrated in glomerulus
and undergo reabsorption and catabolism but
not secreted by the tubulus. Cystatin C detect
the development of AKI at one or two stages
earlier than serum creatinine level based on
ADQI/RIFLE criteria and it increases faster
with contrast media exposure compared to serum
creatinine. However, the use of cystatin C is
uncommon because it is expensive, and lacking
the cut-off point for evaluating AKI. Serum
creatinine, cystatin C and urine output mainly
Vol 44 • Number 3 • July 2012
demonstrate the function of renal excretion, and
they are indirect markers for renal disorders.1
N-ASETIL-β-GLUOSAMINIDASE (NAG)
N-asetil-β-glukosaminidase (NAG) increase
is a key diagnostic indicator in some tubularspecific disease and could identify patients who
have higher risk for GFR decrease compared to
other renal disease.8 NAG is a lysosomal enzyme
in proximal tubular. NAG increase has been
reported in nephrotoxic drugs exposure, delayed
renal allograft function, chronic glomerular
disease, diabetic nephropathy, and it is also
sensitive to detect AKI in critically-ill adult
patients, which may precedes the increase of
serum creatinine level by 12 hours to 4 days. The
higher urine NAG concentration in those patients
diagnosed with AKI, the higher probability for
patients to experience dialysis or death incident.
The advantages of using NAG in AKI is the
sensitivity. Disruption in epithelial brush border
of proximal tubular cell causes NAG to be
released to the urine and the amount of enzyme
could be irectly correlated with tubular disruption.
Quantitatively, it is easy and reproducible with
enzymatic examination using spectrophotometer
to test the specimen with colorimetric. NAG
increase could be found in some condition
without clinical AKI, such as rheumatoid
arthritis, due to analgesic use, non steroid antiinflammatory drug (NSAID), Disease Modifying
Anti Rheumatoid Drugs (DMARDs), secondary
amyloidosis, and vasculitis. NAG increase is also
found in impaired glucose intolerance, which
might be caused as adverse effect of filtered
plasma protein through glomerular capillaries in
tubular cell and hyperthyroidism.5,9,10
RETINOL BINDING PROTEIN
Retinol Binding Protein is a protein with 21
kDa molecular weight, synthesized by liver and
also has important role in transporting vitamin
A from liver to tissue. It is filtrated freely by
glomerular, and undergoes reabsorption in
proximal tubulus. RBP increase is an early
diagnostic tool for nephrotoxicity induced
by cisplatin, lead, mercury, cadmium, and
cyclosporine. Serum RBP level decreases in
patients with vitamin A deficiency; therefore,
urine test might show false negative result.5 The
test is non-invasive enzymatic analysis, which
Biomarker Acute Kidney Injury
is sensitive and specific, and one of the early
indicators of tubular dysfunction. Compared
to other low-molecular weight protein, RBP
has advantages such as relatively constant
production, and no abnormal clinical condition
reported associated with increase production
and its abnormal level in urine. Moreover, it is
stable in urine pH.11 The excretion of retinol
binding protein in urine increases with reflux
nephropathy in children. In diabetic patient, albeit
no albuminuria, retinol binding protein and NAG
excretion in urine can be found.8
NEUTROPHIL GELATINASE-ASSOCIATED
LIPOCALIN (NGAL)
NGAL in human initially was identified as
25 kDa protein which was covalently bound
to neutrophil-gelatinase. Although NGAL is
expressed only in low amount in several human
tissues, it could be induced significantly in
damaged epithelial cells, including the kidney.
Proteomic analysis proves that NGAL is one of
the highly induced protein in kidney following
the ischemic AKI and nephrotoxic in laboratory
animal. A cross-sectional study demonstrated
that AKI patients (with two-fold increases of
serum creatinine level) have significant NGAL
increase in their urine and serum. NGAL level in
urine and serum is correlated to serum creatinin
level and the increase precedes increased
serum creatinine level. Kidney biopsies in AKI
patients showed intense accumulation of NGAL
immunereactively in cortical tubular, indicating
NGAL as a sensitive index in establishing AKI.4
Several studies showed NGAL as early
diagnostic biomarker in AKI, such as a study
in children who had undergone elective cardiac
surgery, which subsequently suffered AKI;
the NGAL serum and urine level increased 10
folds. NGAL is also used as biomarker in kidney
transplantation. There was a correlation between
NGAL staining intensity and delayed graft
function. NGAL is also a predictive biomarker in
nephrotoxicity after contrast exposure. Because
of its highly predictive nature, NGAL was also
used in interventional studies such as hydroxylethyl starch use in albumin or gelatin in elderly
to preserve renal function, it was assessed
whether the NGAL concentration in urine will
be decreased.12
Plasma NGAL were filtered in glomerulus,
and many undergone reabsorption by proximal
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Sri Suryo Adiyanti
tubular; therefore, NGAL excretion in urine only
occurs when there is proximal tubular damage
which disrupts NGAL reabsorption or increase
NGAL synthesis. Genetic expression studies
in AKI showed rapid and massive increase of
NGAL mRNA (up to 1000 folds) in ascending
tubules of Henle loop and collecting tubular.
Therefore, resultant synthesis of NGAL protein
in distal nephron and its secretion to urine seem
to be the largest contribution to the presence of
NGAL in urine. Now, it has been known that AKI
also causes significant increase of NGAL mRNA
expression in certain organs, especially liver and
lung, and NGAL protein which experiences overexpression and released into circulation that can
be a systemic pool. Furthermore, GFR decrease
due to AKI will decrease NGAL clearance,
which causes NGAL accumulation in systemic
circulation, but the mechanism has not been
clearly defined.4
The measurement of plasma NGAL could be
affected by various factors, such as chronic kidney
disease, chronic hypertension, systemic infection,
inflammatory condition, and malignancy. In
patients with chronic kidney disease, NGAL
level is correlated with the severity of kidney
damage. This may become the shortcoming for
determination of NGAL.4,12
INTERLEUKIN-18 (IL-18)
IL-18 mediates ischemic acute tubular
necrosis in mice. The pre-clinical observation
showed that IL-18 in urine has a potential
as AKI biomarker in human. IL-18 is a
pro-inflammatory cytokine which increased
in endogenous inflammation process and was
important in sepsis pathophysiology. IL-18 level
increased significantly in AKI patients compared
to pre-renal azotemia, urinary tract infection,
chronic renal insufficiency, and nephrotic
syndrome. Studies in human showed that IL-18
concentration increased in 24-48 hours prior to
AKI based on RIFLE criteria. The IL-18 urine
level is correlated with AKI severity and it is a
predictor for AKI and mortality in a heterogenic
group of children with critical illness. The IL18 urine level increases 2 days earlier, which
precedes the increase of creatinine level in
non-sepsis patients. The IL-18 urine level has
sensitivity and specificity >90% in diagnosing
AKI. It is a promising marker to be developed
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further since it is rapid, reliable, and affordable
test. Compared to other markers, IL-18 has great
advantages as it could be measured rapidly with
ELISA method.2,5,13
NETRIN – 1
Netrin-1 can be used as AKI biomarker.
Netrin-1 is a 50-75 kD laminin-like protein,
previously known as chemotropic and cell survival
factor in the development of nervous system
and possibly has a role in neovascularization,
cell adhesion and tumorigenesis.3 According
to Ramesh et al.14, netrin-1 was excreted in
urine as early as 1 hour following the functional
injury and will rise 30-40 folds in 3 hours and
reach its peak in 6 hours after the insult. The
study used the mouse kidney injury models,
including ischemia-reperfusion and three toxic
impact of the following substances cisplatin,
endotoxin, and folic acid (in high doses). All of
these were shown to induce netrin-1 excretion.
This observation clearly indicates netrin-1 as a
universal biomarker for hypoxic injury and toxic
renal disorder; therefore, it may be potentially
applicable to various types of AKI, including
cases with unknown causes of AKI.
The significant meaning of netrin-1 excretion
induced by renal dysfunction made a greater value
compared to other markers, including neutrophil
gelatinase-associated lipocalin (NGAL),
interleukin-18 (IL-8), kidney injury molecule-1
(KIM-1), N-acetyl-β-glucosaminidase (NAG),
cysteine rich protein 61 (CYR 61), hepatocyte
growth factor (HGF), meprin A betaandexosomal
Fetuin A. How netrin-1 reacts in AKI differs with
the previous markers because netrin-1 urine level
will return to normal range during reperfusion,
which indicates that netrin-1 could also be used
as a prognostic marker for renal recovery and has
increased value in clinical application.
Netrin-1 has a tendency to be excreted rapidly
following the occurrence of renal dysfunction.
The exact mechanism requires further studies.
Reeves et al14 showed that renal dysfunction will
cause increase of netrin-1 in epithelial tubular
cells, but it did not induce netrin-1 mRNA.
Therefore, increase excretion of netrin-1 in
urine seems to be most likely due to induction
of protein synthesis and release of pre-synthesis
protein. Netrin-1 facilitates cell proliferation and
regeneration after dysfunction, and could be used
Vol 44 • Number 3 • July 2012
as marker for recovery.3
KIDNEY INJURY MOLECULE-1
KIM-1is a cell membrane glycoprotein type
1 which contains 6-cystein domain resembling
immunoglobulin. KIM-1 mRNA increases
higher than other genes following the renal
dysfunction. Such gene expression increases
in 24-48 hours after ischemic events in mice.
KIM-1 gene and its protein expression are not
detected in normal kidney. KIM-1 function in
kidney is to make epithelial cell recognizes
and phagocytes dead cells in kidney due to
ischemia and causing lumen obstruction that
lead to AKI. Epithelial cell undergone apoptosis
will express phosphatidylserine (PS), which
will be recognized by living cells with KIM1 as phagocyte receptor for PS, and will be
phagocyted. Thus, epithelial cell with KIM-1
works as semi-professional phagocyte. KIM-1
is highly specific and sensitive in identifying
toxic substances in proximal tubular. From
kidney biopsy specimen of 6 patients with acute
tubular, KIM-1 urine level increased in 12 hours
after renal dysfunction, preceding the cylinder
formation.5,15,16
CLUSTERIN
Clusterin is the first glycoprotein isolated
from sheep’s testis and it was named due to
its ability to produce clusters of Sertoli cells.
Clusterin is produced in kidney and urine of the
mice, dog, and primate after various pre-clinical
AKI, unilateral ureter obstruction, or subtotal
nephrectomy.Clusterins, just like KIM-1, are
expressed in damaged tubular cells and are
induced in polycystic kidney disease and renal
carcinoma. The main form of human clusterin
(nCLU) is pro-apoptotic and its secretoric
form (sCLU), which increases as a response to
molecular stress, has anti-apoptotic and prosurvival characteristics. Some discoveries of
drugs with the target of sCLU expression might
be a promising therapy for cancer, especially
cancer with sCLU over-expression such as
kidney, prostate, colon, breast and lung cancer.
However, until now, there is no clinical study
indicating that clusterin may be used as a
prognostic indicator or early diagnostic tool of
AKI in human.5
Biomarker Acute Kidney Injury
SODIUM/HYDROGEN EXCHANGER ISOFORM
Sodium/Hydrogen exchanger isoform
(NHE3) is the most abundant sodium transporter
in renal tubulus, which has important role in
proximal reabsorption of 60-70% sodium and
bicarbonate filtered in mice’s kidney. NHE3 are
located in apical membrane and intra-cellular
vesicular compartment of proximal tubular
cells. NH3 excretion in urine is a useful marker
to differentiate control group, patients with prerenal azotemia, and acute glomerular disease, or
acute tubular necrosis. However, it was reported
that if specimen handling and processing were not
optimal, NH3 could be degraded and decreased
NH3 level may be found.5
FETUIN A
Fetuin A is an acute-phase protein synthesized
in liver and secreted into circulation, which has
important role in several different functions, such
as bone resorption, regulation of insulin activity,
hepatocyte growth factor and inflammatory
responses. Fetuin A urine level is found to be
increased in ICU patients with AKI compared
to those without AKI and healthy volunteers.
Immunohistochemical coloring localized fetuin-A
in cytoplasm of damaged proximal tubular cells
with higher concentration in consistent with the
severity. Although the function of fetuin-A in
AKI has not been clearly defined, it might have
important role in tubular cell apoptosis.5
Table 2. Comparison of several AKI biomarkers between
AKI patients and healthy volunteers17
Biomarker
AUCROC
Cutoff
Sensitivity
Specificity
Cystatin C
IL-18
0.85
37
45%
92%
0.83
2.74
68%
95%
KIM-1
NAG
0.95
1.73
80%
90%
0.85
0.015
80%
NGAL
65%
0.89
82.7
80%
96%
Table 3. Comparison of AKI biomarkers between patients
with AKI and without AKI17
Biomarker
AUCROC
Cutoff
Sensitivity
Specificity
Cystatin C
IL-18
0.90
0.11
78%
94%
0.85
2.30
69%
92%
KIM-1
NAG
0.95
0.70
90%
96%
0.85
0.007
99%
100%
NGAL
0.89
83
80%
98%
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SUMMARY
AKI is a frequent condition and associated
with significant morbidity and mortality. In
efforts to detect AKI at early stage, several
biomarkers have been discovered; some are
promising such as KIM-1, NGAL, IL-18,
Clusterin, and others. Cystatin C is a biomarker
for glomerular filtration function; while β2microglobulin, α1-microglobulin, NAG, RBP,
IL-18, NGAL, Netrin-1, KIM-1, Clusterin,
Sodium Hydrogen Exchanger Isoform and
Fetuin A are biomarkers for tubular reabsorption
function. The use of single biomarkers might
not be adequate to determine whether AKI due
to kidney heterogeneity or various dysfunction
conditions. Further studies are needed on the use
of biomarker panel for detecting, monitoring, and
determining the prognosis of AKI. The following
is a summary table of some biomarkers that
has been reviewed in this paper including its
measurement methods.
CONCLUSION
AKI biomarkers might be components of
serum or urine. Several biomarkers have been
discovered, some of them seem to be promising
such as kidney injury molecule-1, netrophil
gelatinase-associated lipocalin, IL-18, sodium/
hydrogen exchange form 3, N-acetyl-β-Dglucosaminidase, and Netrophil gelatinase-
associated lipocalin and kidney injury molecule.
The use of single biomarker might not be
adequate to determine whether AKI due to renal
heterogeneity or various dysfunction conditions.
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Renal function
Method of measurement
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Tubular
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Glomerulus
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Clusterin
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Tubular
ELISA
NGAL
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Tubular
ELISA, Luminex based
assay
NHE3
The most abundant sodium transporter in tubular,
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Tubular
Immunoblotting
Exosomal Fetuin A
Acute-phase protein synthesized in liver, sample
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Tubular
Immunoblotting
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Description
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